The present invention relates to a free-piston Stirling engines provided with piston and displacer positioning devices and more particularly to those Stirling engines provided with center porting devices, gas springs and spin lubrication.
As the piston and displacer of a free-piston Stirling engine receprocates, there tends to be a nonuniform leakage of gas from one space to another which, if uncontrolled, causes an average pressure increase in the space into which the gas leaks, thereby pushing the piston or displacer one way or another within the engine housing. This occurs between the flow of gas through an annular gap between a cylindrical wall and a moving cylindrical surface is not sinusoidal: it is greater at high pressure than at low pressure. Gas will leak out of a space having a higher pressure peak into a space having a lower pressure peak, and more gas will leak out during the high pressure differential than will leak in during the low pressure differential.
Center porting provides a corrective flow of gas when the displacer or piston is at the center position. In this manner excessively low and excessively high pressure differentials in a particular space may be avoided by providing venting to relieve excessively high pressures and an influx of gas to raise excessively low pressures.
In the past, proper positioning of reciprocating elements has been maintained by providing valves or passageways in the engine housing wall or by providing periodic fluid engagement between a space undergoing substantial pressure variation and a reference space in which the gas pressure is maintained at or about the average pressure of the system.
U.S. Pat. No. 3,899,888 issued Aug. 19, 1975 to Shuman discloses a piston positioning means comprising vertical grooves formed in the center portion of the cylinder wall within which the piston reciprocates. The grooves act as by-pass passageways having a fluid flow impedance that is substantially the same in both directions. As the piston reciprocates, leakage of gas through the grooves tends to maintain the center of piston oscillation near the center of the grooves. A major drawback of this arrangement is that a close tolerance fit must be provided between the cylinder and piston walls so that fluid communication with the grooves is established at the proper piston position. A close fit requires expensive machining or honing of the inner surface of the engine wall. In the present invention, in contrast, a sloppy fit may be provided between the piston and housing and inexpensive piston rings provided therebetween because the primary bearing surfaces are provided by the outer surface of a center rod and the inner cylindrical walls of the piston and displacer. Since the openings in the reciprocating elements are substantially smaller in diameter than the engine housing, it is less expensive to provide a close tolerance fit between center rod and inner piston wall in the present invention than between the outer piston wall and engine housing in the Shuman patent.
U.S. Pat. No. 4,183,214 issued Jan. 15, 1980 to Beale et al. discloses a displacer provided with a cylindrical bore and a gas spring compartment formed therein. A rod is fixed to the engine housing at one end by means of a spider. The other end is slideable in the cylindrical bore provided in the displacer. Center porting passages are provided in the displacer and the stationary rod so that the central position of the displacer can be maintained. A major drawback with this arrangement is that the spider is an impediment to the movement of the piston and displacer. These reciprocating elements tend to come together at a certain point in the Stirling cycle. Since the spider maintains a fixed position between piston and displacer, it is impossible to have an overlap of the piston and displacer strokes, thus lowering power and efficiency. In the present invention, a stationary center rod is fixed to the housing at a point which does not interfere with the movement of the reciprocating elements. In addition, the present center rod arrangement permits the piston and displacer to rotate in order to provide for spin lubrication. In the earlier Beale et al. patent, however, the spider would interfer with and eventually be mangled to a spinning piston or displacer.